Finishes on ball grid array (BGA) bond pads changed from nickel to copper. The copper finishes tend to oxidize. A thin organic layer of organic solderability preservative (OSP) may be applied to the copper bond pad to prevent oxidation. The OSP bonds to the bond pad surface and protects the copper until soldering. However, cleaning copper OSP may be difficult due to upstream in-process contaminants.
BGAs may use a two-step reflow process to form a ball attach joint. The first step involves cleaning the OSP layer atop the underlying copper bond pad. A first step may print and reflow water soluble (WS) flux over the bond pad. Water jets then may wash the WS flux residue off the bond pad surface during the deflux process which immediately follows the first step. A second step may apply a no clean (NC) flux to the cleaned bond pad surface during a ball attach (BA) operation. The NC flux retains the solder ball and improves wetting during solder ball reflow leading to formation of the actual BGA joint.
Residue from the BA process may stain a second level interconnect (SLI) interface of the BGA package. The residue may be a remnant of the NC flux spreading out over the bond pad. Cosmetic defects from the residue may result in final visual inspection (FVI) rejections.